Not Applicable.
Not Applicable.
1. Field of Invention
The present invention relates generally to power conversion electronics and, more particularly, to current limit circuits for interleaved power converters.
2. Description of the Background
DC-to-DC power converters are power processing circuits that convert an unregulated input DC voltage to a regulated DC output voltage. Switched-mode DC-to-DC power converters typically include an inverter, a transformer having a primary winding coupled to the inverter, and a rectifying circuit coupled to a secondary winding of the transformer. The inverter typically includes a pulse width modulated (PWM) switching device, such as a field effect transistor (FET), that converts the DC input voltage to an alternating voltage, which is magnetically coupled from the primary winding of the transformer to the secondary winding. Conduction of the PWM switching device is regulated by a PWM control circuit. The rectifying circuit rectifies the alternating voltage on the secondary winding to generate a desired DC output voltage. The DC output voltage of the power converter, which is used to power a load, is typically regulated by varying the duty cycle of the PWM control signal from the PWM control circuit applied to the control terminal of the PWM switching device.
If the load develops a short or otherwise draws too much current, an overload condition occurs that could damage the load or the converter. Consequently, overload protection, which limits the maximum output current of a power supply in order to protect the load and/or power supply, is a typically required element of a power converter. Overload protection is ordinarily realized with a current limit circuit. Typically, a current limit circuit senses the peak current on the primary side of the power converter. When an overload condition occurs, the current limit circuit outputs a signal to the PWM control circuit, causing the PWM control circuit to reduce the duty cycle of the PWM control signal supplied to the PWM switching device.
In theory, this approach limits the output current of the converter to a safe value. However, current limiting based on peak current control in a buck-boost type converter results in an overload response characteristic that is one of constant power. To effectively safeguard the power converter unit during an overload condition, therefore, additional control circuitry is needed. The functions of the additional control circuitry, however, typically result in a non-constant current response and are prone to several control problems.
In addition, it is difficult to effectively implement overload protection in interleaved mode converters. Interleaved, or multi-phase, power converters commonly include a set of two or more identical smaller power converters. These smaller converter xe2x80x9ccellsxe2x80x9d are connected so that the output of the resultant larger converter represents a summation of the outputs of the individual cells. The cells are typically operated at a common frequency, but with the phase shifted between them so that the conversion switching occurs at regular intervals.
One important consideration in a multi-phase arrangement is the current balance between the parallel-connected converter cells. For example, if a power supply has two parallel-connected converter cells, typically each converter is designed to deliver half of the load current. In the event that one of the converters is not delivering half of the load current, the other converter has to xe2x80x9cmake up the difference.xe2x80x9d A converter that is forced to compensate for another converter may not be able to provide sufficient current to the load. In this event, the load equipment may not function properly at all.
Accordingly, in addition to overload protection, interleaved converters also require current sharing control circuitry. It is difficult to implement overload protection, however, in interleaved mode converters because the total output current is the sum of the output current of the individual converter cells. As such, the conduction interval of the rectifying circuit of one converter cell typically extends through the storage/conduction interval of another of the converter cells. Conventional techniques to implement current mode control and overload protection for interleaved converters, however, require current limiting and current sharing control circuitry for each of the individual converter cells, which is costly and inefficient.
Accordingly, there exists a need in the prior art for a technique to implement constant current overload function for interleaved buck-boost converters that is economical and efficient. There further exists a need for such a technique to be realized in a single circuit, and which inherently forces each converter cell in a multi-phase arrangement to share the total load current.
The present invention is directed to a current limit circuit for a power supply including first and second interleaved power converters. According to one embodiment, the current limit circuit includes a peak detection circuit connected to a current sensing circuit of the power supply, a sample/hold circuit connected to the peak detection circuit, and an averaging filter circuit connected to the sample/hold circuit.
According to another embodiment, the present invention is directed to a control circuit for a power supply including first and second interleaved power converters, wherein the first power converter includes a first pulse width modulated switching device and second power converter includes a second pulse width modulated switching device. According to one embodiment, the control circuit includes a current sensing circuit for sensing an input current of the first and second interleaved converters, a current limit circuit connected to the current sensing circuit for generating an output signal having a voltage value that is proportional to an average output current from the first and second interleaved power converters, a voltage control amplifier responsive to an output voltage of the power supply, and a pulse width modulated control circuit having an input terminal and first and second output terminals, wherein the input terminal is connected to both an output terminal of the current limit circuit and the output terminal of the voltage control amplifier, and wherein the first output terminal is connected to a control terminal of the first pulse width modulated switching device and the second output terminal is connected to a control terminal of the second pulse width modulated switching device.
According to another embodiment, the present invention is directed to a power supply. The power supply includes a first power converter including a first pulse width modulated switching device, a second power converter parallel-connected to the first power converter and including a second pulse width modulated switching device, a current sensing circuit for sensing an input current of the first and second converters, a current limit circuit connected to the current sensing circuit for generating an output signal having a voltage value that is proportional to an average output current from the first and second power converters, a voltage control amplifier responsive to an output voltage of the power supply, and a pulse width modulated control circuit having an input terminal and first and second output terminals, wherein the input terminal is connected to both an output terminal of the current limit circuit and the output terminal of the voltage control amplifier, and wherein the first output terminal is connected to a control terminal of the first pulse width modulated switching device and the second output terminal is connected to a control terminal of the second pulse width modulated switching device.
The current limit circuit of the present invention provides a constant current overload function for interleaved converters of, for example, the buck-boost derived type. In contrast to the prior art, the current limit circuit of the present invention may be realized in a single circuit that inherently forces each interleaved converter in a multi-phase arrangement to share total load current. According to one embodiment, the current limit circuit of the present invention develops a signal proportional to the average output current for an interleaved power supply. This signal may be used to provide average current mode control or overload protection with a constant current characteristic.
These and other benefits of the invention will be apparent from the detailed description hereinbelow.